This invention relates to data processing systems, and particularly to data processing systems of the type having a plurality of subsystems linked by a star coupler.
Star couplers are well known in data processing technology. Systems employing star couplers are described, for example, in Metcalf & Boggs, "Ethernet: Distributed Packet Switching for Local Computer Networks," 19 Communications of the ACM 395 (July, 1976); Rawson & Metcalf, "Fibernet: Multimode Optical Fibers for Local Computer Networks," 26 IEEE Transactions on Communications, 983 (July, 1978); Data processing System Having Dual Channel System Bus, Gunderson, et al. U.S. Pat. No. 4,417,334, Nov. 22, 1983; and Data Processing Subsystem Wherein At Least One Subsystem Has a Local Memory and a Mailbox Memory within the Local Memory for Storing Header Information, U.S. Pat. No. 4,387,441, Kocol, et al., June 7, 1983.
The latter two patents are assigned to the Assignee of this invention. The present invention is usable in the systems disclosed in these patents, which are incorporated herein by reference.
When used in a data processing system having a plurality of stations or terminals, a star coupler is typically connected to pairs of transmission lines, with one pair associated with each subsystem. A first transmission line of the pair carries signals away from the subsystem to the star coupler, and a second transmission line of the pair carries signals from the star coupler to the subsystem. (It should be noted that each transmission "line" comprises a twisted pair of wires for differential signal transmission.) When any subsystem transmits or generates a signal or message, that message is received by the star coupler from the first transmission line associated with the subsystem and is directed or passed to every subsystem by way of each second transmission line, including the second transmission line returning to the subsystem that transmitted the message. This of course offers advantages in linking multiple subsystems, since a subsystem transmitting a message receives back the message at the same time each of the other subsystems receives the message. The transmitting system can therefore check for any transmission errors, without requiring a receiving subsystem to regenerate the message.
The function of the star coupler is essentially to take all the signals received on the input side of the coupler on the first transmission lines, to logically OR all these first transmission line signals, and to redrive the resulting signal to all terminals or subsystems over the second transmission line of each pair.
A potentially serious problem resides in the use of a star coupler in this mode to logically OR all the incoming signals on the first transmission line. If the incoming data from any transmission line suffers from a high bit error rate, these errors can be quickly propagated throughout the system. This is especially true where noise is picked up from one system and its associated transmission line.
It has been found that communication may be seriously impacted when any non-star station processor is powered down. Some noise may continue to be coupled via low impedance output of the power supplies to the transmitting amplifiers at the terminal processor. Such noise can be transmitted through the cable to the star coupler, and retransmitted to the other stations until detected as errors, slowing the processing rate of the entire system.
A similar problem arises when any station processor is physically disconnected from the star coupler. When a station is disconnected, the cable carrying the first and second transmission lines no longer has its shield grounded. As a result, spurious noise may be picked up by the wires and the normally grounded shield and conveyed to the receiving amplifier at the star coupler to be injected into the system and retransmitted.